In general, submicroscopic gas bubbles with diameter less than 1 μm (1000 nm) are called “nanobubbles,” whereas microscopic gas bubbles with diameter equal to or greater than 1 μm are called “microbubbles.” The nanobubbles and microbubbles are distinguished from each other.
Patent Document 1 describes microscopic gas bubbles (microbubbles) characterized for having diameter less than about 30 μm upon their generation at normal pressures; gradually miniaturizing over a predetermined lifespan; and vanishing or dissolving thereafter.
The Patent Document 1 also describes examples and their results of applying the microbubble characteristics such as gas-liquid solubility, cleaning function or bioactivity enhancement to improve water quality in closed bodies of water such as a dam reservoir, enhance the growth of farmed fish and shellfish or hydroponic vegetables and the like, and sterilization or cleaning of organisms.
Patent Document 2 describes a method for generating nanobubbles with diameter less than 1 μm by decomposing part of liquid therewithin. Also Patent Document 3 describes a method and an apparatus for cleaning objects using nanobubble-containing water.
Patent Document 4 describes a method for producing nanobubbles by applying physical stimulation to microbubbles in liquid to thereby rapidly reduce the bubble size. Furthermore, Patent Document 5 describes a technology according to oxygen nanobubble water consisting of an aqueous solution comprising oxygen-containing gas bubbles (oxygen nanobubbles) with 50-500 nm diameter, and a method to produce the oxygen nanobubble water.
As described above, nanobubbles have not only the microbubble functionalities, but also excellent engineering functionalities to directly affect organisms in their cellular level, allowing a broader range of applications, such as semiconductor wafer cleaning and dermatosis treatment, than that of microbubbles and nanobubbles are expected to have even higher functionalities in the future.
Patent Document 1: JP-A-2002-143885
Patent Document 2: JP-A-2003-334548
Patent Document 3: JP-A-2004-121962
Patent Document 4: JP-A-2005-245817
Patent Document 5: JP-A-2005-246294
It has been verified that the nanobubbles described above are generated instantaneously when microbubbles collapse in the water, and are known for their extremely unstable physical characteristics. Therefore it is difficult to put nanobubbles to practical use by stably producing and retaining them for an extended period of time.
For this reason, the Patent Document 3 is suggesting to generate nanobubbles by applying ultrasonic waves to decomposed and gasified solution. However, ultrasonic generators are expensive, large-sized and difficult to use and perform matching, prohibiting their wide use.
Also the Patent Document 1 discloses a method and an apparatus for generating microbubbles by force feeding liquid into a cylindrical space in its circumferential direction to create a negative pressure region, and having the negative pressure region absorb external gas. However, this apparatus only generates microbubbles, and does not stably produce nanobubbles with smaller diameter.